StarFish 453OEM
System Manual
12
4.2.
What Does A Side-Scan Sonar Image Look Like?
The figure below is a real image captured by a StarFish sonar, where each horizontal line is a representation of
time versus the intensity of the reflected echoes. The further something is away from the centre line of the display,
means the longer it took for the echo to be received.
In this figure, both the port (left) and starboard (right) channels are operating at the same time. The sonar is
positioned in the middle of the topmost line, and as the sonar is moved through the water the image will scroll
away from it as more data is added.
4.3.
Calculating Depth Below The Sonar (Altitude)
A useful measure to know when using a side-scan sonar is the height above the seafloor, so a safe distance can be
maintained and hazards avoided.
Looking at the previous image, we can see a large
black “hour
-
glass” shaped area in the middle of the display,
where no echoes have been received.
In fact this area is showing us the height the sonar is above the sea
bed.
As the figure opposite shows, the nearest object to the sonar will
always be the seabed below it (assuming a relatively flat seabed).
However, it will take some time for the “bottom echo” to be
returned, and as the display shows the received echoes from when
the transmission started, this area appears black.
Therefore, the narrower the band of black is, the closer the sonar is to the seabed
–
and as the image is built up
over time, if the sonar’s altitude changes we actually start to see the profile of the seabed that the sonar has
passed over.
This means in the figure above, the seabed gets shallower in the middle of the display, and then gets deeper again
at the bottom.
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